Flow sensors are utilized in a variety of fluid-sensing applications for detecting the quality of fluids, including gas and liquid. Flow sensors for such fluids, which detect the fluid flow or property of fluid, can be implemented, for example, as sensors on silicon in microstructure form. For convenience sake, and without limitation, the term “flow sensor” not only can refer to the flow of a fluid (including both gas and liquids), but can also be implemented in the context of thermal sensors. The reader will appreciate that such sensors may be also utilized to measure primary properties such as temperature, thermal conductivity, specific heat and other properties; and that the flows may be generated through forced or natural convection.
A fluid or thermal-type flow sensor typically includes a substrate that includes a heating element and a proximate heat-receiving element or two. If two such sensing elements are used, they are preferably positioned at upstream and downstream sides of the heating element relative to the direction of the fluid (liquid or gas) flow to be measured. When fluid flows along the substrate, it is heated by the heating element at the upstream side and the heat is then transferred non-symmetrically to the heat-receiving elements on either side of the heating element. Since the level of non-symmetry depends on the rate of gas flow and that non-symmetry can be sensed electronically, such a flow sensor can be used to determine the rate and the cumulative amount of the fluid flow.
Such flow sensors generally face potential degradation problems when exposed to harsh (contaminated, dirty, condensing, etc.) fluids, including gases or liquids that can “stress” the sensor via corrosion, radioactive or bacterial contamination, overheating, or freeze-ups. The sensitive measurement of the flow, or pressure (differential or absolute) of “harsh” gases or liquids that can stress corrode, freeze-up, or overheat the sensing elements is a challenge that is either unmet or met at great expense.
Among the solutions proposed previously are passivation with the associated desensitization of the sensor, heaters to avoid condensation or freeze-ups (or coolers to prevent overheating) at the expense of sensor signal degradation, cost increase and possible fluid degradation, or filters to remove objectionable particulate matter. Frequent cleaning or replacement of the sensors is an additional, but costly, solution. Sensitive, membrane-based differential pressure sensors can be protected against contamination because no flow is involved, but they are much less sensitive and much more expensive than thermal micro-sensors, in addition to not being overpressure proof.
Another problem with fluid or thermal flow sensors, particularly, micro-bridge thermal flow sensors, it that such devices possess a relative small dynamic range. A micro-bridge thermal flow sensor, for example, typically incorporates a heater that consumes a great deal of power. It is therefore difficult to adapt such devices for passive wireless applications. An additional problem with thermal flow sensors is that heating may introduce unwanted chemical compositional change or damage the fluid (e.g., blood), particularly in medical applications.
It is believed that one possible solution for the problems inherent with flow sensors is the use of acoustic wave devices, such as, for example, surface acoustic wave sensor. Examples of acoustic wave sensors include devices such as surface acoustic wave sensors, which can be utilized to detect the presence of substances, such as chemicals. An acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor.
Surface acoustic wave (SAW) devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers (IDTs) placed on a piezoelectric material. A SAW device relies on the use of waves that propagate at the surface of a piezoelectric substrate, such that the displacement amplitudes of such waves experience an exponential decay beneath the surface. A SAW sensor can be fabricated on a very thin diaphragm. The theoretical inherent strengths of elastic materials are generally orders of magnitude greater than the measured strengths of the ordinary forms of these materials. This reduction in strength is known to be caused principally by surface flaws, such as scratches, which concentrate the applied stress and thereby lead to fracture at loads which are much lower than the theoretical maximum.
Etching has been used in BAW device manufacturing for many years, such as inverted mesa type high frequency fundamental crystal resonators. Etching was introduced in SAW sensor manufacturing to etch off the backside of the sensor. Etching in a wet chemical process has been shown to be capable of chemically polishing quartz over a broad range of conditions. The etching process can remove a large amount of materials from lapped blanks while simultaneously producing an improved surface finish, without producing shifts in the angles of cut. This process can also produce SAW dies with greater strength, which is particularly important for many sensing applications.
Etching does not introduce defects, but etching does reveal defects. Swept quartz is needed for micro-machined SAW sensors. A swept quartz wafer is usually free of etch channels. Etch channels are a consequence of dislocations at which impurities are segregated. Swept quartz also has fewer etch pits compared to non-swept quartz. SAW sensors can also be constructed using double side polished wafers. For example, a polished device may possess a mechanical strength increased by 10 times. This is critical in overpressure proofing. Because the backside of the SAW wafer is to be etched, the surface condition is critical in the etch surface finish. The quality of an etched surface finish primarily depends on the surface prior to etching, the depth of etch, etchants selection, speed of etching and the quality of quartz utilized.
Surface acoustic wave devices may have either a delay line or a resonator configuration. The change of the acoustic property due to measurand can be interpreted as a delay time shift for the delay line surface acoustic wave device (SAW-DL) or a frequency shift for the resonator (BAW/SAW-R) acoustic wave device.
Acoustic wave sensing devices often rely on the use of quartz crystal resonator components, such as the type adapted for use with electronic oscillators. Acoustic wave devices are attractive to fluid flow applications because of their high sensitivity, resolution and ruggedness. The detection mechanism implemented depends on changes in the acousto-elastic properties of the piezoelectric crystal when exposed to a gas or fluid. Wired measurement results are usually obtained as the output frequency of a loop oscillator circuit, which utilizes the acoustic wave device as the feedback element.
In general, existing SAW flow sensor concepts are based on the same thermal mass flow theory that applies to micro-bridge thermal flow type sensors, which measures the displacement of the temperature profile caused by the flow around a heater. One of the problems with existing SAW flow sensor designs is that such configurations incorporate two or more SAW temperatures sensors along with a heater. Such a design increases the sensor size and raises the production cost and power usage thereof. Additionally, the use of a heating element introduces accelerated aging for any IDT electrodes and associated SAW sensing components.
There are two other techniques for measuring flow rate using an acoustic wave sensor. One is to measure the flow-induced shear stress. This kind of acoustic wave sensor could have a textured surface. The textured surface helps to increase the shear stress induced by flow. The other technique involves measuring the pressure difference between two points by applying Bernoulli's law.
Shear stress is a flow-induced force exerted on the wall of the flow tube. This force relies upon units of force per unit square area. The flow rate can be calculated from the following relationship: Q=τ/6μ, where Q is the fluid flow rate (m3/sec), τ is the shear stress (N/m2), μ is the fluid viscosity (Nm/s2), and A is related to the geometry of the tube.
The magnitude of the fluid flow rate is linearly dependent on the shear stress. Since the fluid viscosity and density are temperature dependent, changes in temperature affect the shear stress, hence the flow rate measurement. The effect of temperature on density is usually small, and when temperature increases, the fluid viscosity usually increases, so the shear force usually decreases. Based on the foregoing, it is desirable to implement an acoustic wave temperature sensor on the same sensor die of an acoustic wave flow sensor.